Etch rate and especially etch uniformity of plasma etch processes, i.e. the question of whether or not in some areas of the wafer the etched layer is removed faster than in others, are usually measured by ex-situ techniques. The layer thickness is measured on a number of points across the wafer, for example by ellipsometry, at the start of the process and at various points in time as the etch progresses. This means that the wafer is taken out of the process chamber for each measurement, until a statistical data set is obtained which allows to derive the etch rate and uniformity. Such techniques are cumbersome and time-consuming.
Interferometry has been used as a way to observe the etching process in real-time. Documents U.S. Pat. No. 7,821,655 and U.S. Pat. No. 6,585,908 show examples of techniques for in-situ measurement of the etch rate or for removal endpoint prediction during plasma etching, by analyzing interference patterns through a side window of the reactor. While focusing on the determination of the etch rate, none of these documents discloses an in-situ technique for monitoring the etch uniformity across the wafer.
U.S. Pat. No. 5,450,205 relates to a technique wherein a CCD camera is mounted opposite the substrate during plasma etching. This technique allows to monitor the etch rate and uniformity across the complete substrate surface by analyzing interference patterns. It is however a technically complex solution, which is furthermore not applicable to capacitively coupled plasma etch reactors wherein a plasma is created between two closely spaced electrodes.